Basic building blocks of semiconductor devices are the junction, essentially a narrow region where the doping level and nature of dopants changes abruptly, and a conductor, used to lead electronic carriers to and from the junction or more complicated elements, comprising one or more junctions. Commonly junctions are fabricated by high temperature thermal diffusion of a dopant from an external source or by direct ion implantation of the dopant. This is so because of the need to reach conditions under which the dopants can move in the semiconductor, so that they can attain the necessary non-equilibrium distribution. At the temperature, where thermal diffusion is commonly carried out, a semiconductor with a doping density that is within the range, normally employed for electronic devices, behaves intrinsically. This means that it has a higher concentration of so called "intrinsic" free charge carriers (these are carriers that are generated by band-to-band transitions) than of carriers generated by dopants. This reduces the possibility to use electrical influence to guide the dopants, due to electrical screening. At lower temperatures, where such use could be beneficial, the dopants have too low diffusity. Ion implantation leads generally to damage in the region that has been implanted, leading to undesirable electrical properties that are removed by subsequent thermal anneal.
Both above methods need an external source, and a surface through which the doping species are introduced into the material, which limits the dimensionality of devices that can be produced in this manner (cf. Handbook on Semiconductors, T. S. Moss ed., vol. 3 "Materials, Properties and Preparation", S. P. Keller, volume ed. (North-Holland, Amsterdam, 1980)).
The effects of electric fields on influencing the motion of external dopants in simple, elemental semiconductors have been described for the cases of Li, Cu, Ag or Au in Ge or Si at elevated temperatures (C. S. Fuller and J. C. Severiens, Phys. Rev. 92 (1953) 1322; 94 (1954) 21; C. J. Gallagher, J. Phys. Chem. Solids 3 (1957) 82; see also B. I. Boltaks, "Diffusion in Semiconductors", Acad. Press N.Y. 1963). Fuller and Severiens studied Li migration in p-Si and p-Ge at .about.350.degree.-900.degree. C., .about.10 V/cm by the p/n junction method, after local thermal in-diffusion of Li (C. S. Fuller, J. C. Severiens, Phys. Rev., 96 (1954), 21). Commercial radiation detectors (T. Miyachi, S. Ohkawa, T. Emura, M. Nishimura, O. Nitoh, K. Takahashi, S. Kitamura, Y. Kim, T. Abe and H. Matsuzawa, Jap. J. Appl. Phys., 27 (1988), 307) resulted from the finding that high resistivity Si was obtained after electromigration of Li into p-Si. Here interstitial Li (Li.sub.i) acts as donor, compensating acceptors. Li profiles in Si were formed at 50.degree.-350.degree. C. by E. M. Pell, J. Appl. Phys., 31 (1960), 291). Pell has shown that under the condition, when a sample (Si, with an n/p junction, created by Li doping) is kept at elevated temperature and an electric field is applied, Li drift leads to the creation of an intrinsic area in the bulk of the Si. Therefore, the initial n/p junction transforms into an n/i/p junction.
Interstitial Cu is also found to be mobile at elevated temperatures in Si, Ge, and GaAs. Cu drift at 250.degree. C. and electric field strength of 2 V/cm was investigated by the radioactive tracer technique in GaAs (R. N. Hall, J. H. Racette, J. Appl. Phys., 35 (1964), 379). Acceptor behavior of substitutional Cu was used for the compensation of shallow donors in GaAs (R. A. Roush, D. C. Stoudt, M. S. Mazzola, Appl. Phys. Lett., 62 (1993), 2670). For modern electronic devices Cu, Li, and the like are undesirable as dopants in Si, for example, as they lead to deterioration of the required electronic properties. An exception is their use in photoconductive devices, which often have a homogeneous distribution of the dopant, and as lifetime killers in transistors to improve switching times. Klingler et al. (Klingler et al. Appl. Phys., A59 (1994) 233) have shown the feasibility of p-n junction creation in TiO.sub.2. In the case of TiO.sub.2 (described also in J. Solid State Chem. 21 (1977) 305 and Solid State Ionics 18/19 (1986) 873) the freezing-in of a smooth linear doping profile, created at high temperature (700.degree. C.) by applying an electric or chemical potential gradient is suggested to create p and n regions.
Freezing-in had to be used here as in classical doping by thermal in-diffusion, since otherwise the compositional gradients that are created will not be stable. Indeed stability is one of the more problematic aspects of using an electric field for doping. This problem can be overcome by the present invention by using high field strengths, since then new, non-equilibrium mechanisms, that change the dopant density, can be activated. The situation can be compared to that of thermal in-diffusion of dopants, where a non-equilibrium state, obtained at high temperature, is frozen in, giving a doping profile that is stable under temperatures where the device works. In contradistinction, the present invention creates a non-equilibrium profile with a strong electric field which is stable at the lower fields used during operation of the device.
Semionics, the types of materials that the invention relates to, have been studied, in terms of their basic properties. One of them, CuInSe.sub.2, is being developed in thin film polycrystalline form, for use in a photovoltaic device. The occurrence of ionic conductivity in a number of semionics, was disclosed in U.S. Pat. No. 4,115,633. The materials were of the type ABX.sub.2 and AB.sub.5 X.sub.8 where A=Cu,Ag; B=In,Ga; X=S,Se. In later work, it was shown that a high electric field, applied to an In/CuInTe.sub.2 /In structure, can cause a change from a low to a high conductivity state. Because of the very high current that flows, thermal effects probably played a dominant role (I. V. Bodnar et al., Neorg. Mater. 20, (1984) 1810). R. J. Matson et al. (Appl. Phys. Lett. 50, (1987) 158) showed that CdS deposition onto CuInSe.sub.2 can lead to partial conversion of the CuInSe.sub.2, from p to n-type. In previous studies of semionics (D. Soltz, M. Sc. Thesis, Feinberg Graduate School, The Weizmann Institute of Science, Israel (1987); D. Soltz et al., Solid State Ionics, 28-30 (1988) 1105) normal classical thermal diffusion (e.g. of Cu) was found to lead to type conversion in CuInSe.sub.2.
In these studies the effective diffusion coefficient of Cu was measured in CuInSe.sub.2, in separate experiments, using a well-known point contact method, without using an external phase. However, attempts to obtain type conversion by this method were not successful, since they led to thermal breakdown of part of the material, due to run-away current flow. The only electric field effects noted in these studies were those obtained close to equilibrium, leading to the smeared-out gradual compositional gradients created during measurement of the diffusion coefficient.
Another type of semionic is (Hg,Cd)Te, a material that is also of interest for its infra-red detection properties. In addition, there are various other semiconducting materials, such as, Cu or Li doped ones, e.g. Si:Li, Ge:Li, CdS:Cu; ZnSe:Li and GaAs:Cu(:Si), that are mostly of interest for optical or opto-electronic applications (photoconductivity, optical switching, light emission, x-ray and x-ray detection) that show semionic behavior. Some analogues of CuInSe.sub.2 (e.g. AgGaSe.sub.2, ZnGeP.sub.2) are furthermore of interest for non-linear optical applications.